How Long Until The Moon Slows The Earth To A 25 Hour Day?

At this rate of the Moon’s gravitational force slowing down Earth's rotation, how long will it take to increase an hour to our day?

NASA's Lunar Reconnaissance Orbiter (LRO) recently captured a unique view of Earth from the spacecraft's vantage point in orbit around the moon. Image Credit: NASA/Goddard/Arizona State University

The Earth’s rotation is indeed being slowed down by the presence of the Moon - every year, the Moon gains a little energy from the Earth, and drifts a little farther away from us. This drift is imperceptible to the human eye, but measurable, with the aid of undertakings like the Lunar Laser Ranging Experiment, which regularly bounces a laser off of a retroreflector that Apollo astronauts placed there.

Both the drift of the Moon and the slowing of the rotation of the Earth are very very small effects- the slowing of the Earth’s rotation over the last 100 years is estimated to be about 1.4 milliseconds. That’s a slowing of 0.0014 seconds total, over 100 years. Another method of estimating the slowing of the Earth uses historical records of solar eclipses to figure out exactly how fast the Earth must have been rotating in the past, and comes up with an average slowing of 2.5 milliseconds each century. To extrapolate out into the future, I’m going to use the average of these two numbers, and guess that we’re dealing with a slowing of approximately 0.002 seconds every century.

As a point of reference, this rate of slowing means that it will take 25,000 years to add a half a second to the Earth’s day. A whole second will take 50,000 years.

The release of the first images from NOAA’s newest satellite, GOES-16, is the latest step in a new age of weather satellites. This composite color full-disk visible image is from 1:07 p.m. EDT on Jan. 15, 2017, and was created using several of the 16 spectral channels available on the GOES-16 Advanced Baseline Imager (ABI) instrument. The image shows North and South America and the surrounding oceans. GOES-16 observes Earth from an equatorial view approximately 22,300 miles high, creating full disk images like these, extending from the coast of West Africa, to Guam, and everything in between. Image Credit: NOAA/NASA

To add an entire hour? Every hour contains 3,600 seconds - (60 minutes to an hour, and 60 seconds to a minute). And so, to wait long enough to gain 3,600 seconds, we’ll need to wait 50,000 years 3,600 times over - 180 million years.

Every so often, a powerful earthquake will strike somewhere on the Earth, and the news will report that the planet’s days have shortened again - but this effect is even smaller! The massive 9.0 Earthquake in Japan a few years back only shortened the length of our day by a microsecond - a thousand times smaller of an effect than the slowing of our days by the Moon. You’d need a thousand earthquakes that strong in a century in order to cancel out what the Moon is doing from afar.

180 million years is a hefty chunk of time. Assuming that we humans have managed to tend to our planet, and prevent any incoming destructive asteroids from hitting us, among other problems, our little star will have made it 75% of the way around our entire galaxy.

I am an Assistant Professor in Physics & Astronomy at Oberlin College. I received my Ph.D in 2014 from the University of Victoria, studying the interactions between galaxies in the nearby Universe. I have been running an outreach blog called Astroquizzical since 2013, ...